HAZOP STUDY

INTRODUCTION:-

                                 The technique of hazard and operatibility studies, or in more common terms HAZOP, has been used and develop approximately decades for identifying potential hazard and operatibility problems caused by deviation from the design intent of both and new and existing process plants. Before processing further, it might be as well to clarify some aspects of these statements.

POTENTIAL HAZARDS AND OPERATIBILITY PROBLEMS

You will note the bold AND in the above handing, it’s because high profile of production plant accident, emphasis is too often placed upon the identification of hazards to the neglect of potential problems. Yet it is the latter area that benefits of HAZOP study are usually the greatest.

INDUSTRIES IN WHICH THE TECHNIQUE IS APPLIED

HAZOP were initially invented by ICI in the united kingdom but the technique only started to be more widely used with in the chemical industries after the Fix borough disaster in which a chemical plant explosion kill 28 people, many if which were ordinary house holders living near by. Through the general exchange of ideas and personnel, the system was adopted by the petroleum industries, which has a similar potential for major disasters. This was then followed by the food and water industry, where the hazard potential is as great, but of a different nature, the concern being more to do with contamination rather than explosions or chemical release.

BASIC CONCEPT:-

Essentially the HAZOP procedure involve taking a full description of process and systematically questioning every part of it to establish how deviation from the design intent can arise. Once identified, an assessment is made as to whether such deviations and their consequences can have a negative effect upon the safe end and efficient operation of plant. If consider necessary action is then taken to remedy the situation.

The critical analysis is applied in a structured way by a HAZOP team and it relies upon them releasing their imagination in an effort to discover credible causes of deviations.

In practice, many causes will be fairly, obvious such as pump failure causing loss of circulation in a cooling water facility mentioned above. However, unlikely they may seem at first consideration. In this way he study much more than a mechanistic checklist type of review. The result is that there good chance that potential failures and problems will be identified, which had not previously been experienced in the type of plant being studied.     

KEY WORDS:-

                           An essential feature in this questioning and systematic analysis in the use of  Key words to focus the attention of the team upon deviations and their possible causes. The key words are divided into two subsets.

Primary key words: which focus upon a particular aspect of the design intent or an associated process condition or parameter.

Secondary key words: which when combined with a primary key word, suggest possible deviations.

PRIMARY KEY WORDS:-

These reflect both the process design intent and operational aspects of the plant being studied. Typical process oriented words might be as follows. The list below is purely illustrative, as the words employed in a review will depend upon the plant being studied.

Flow	Pressure
Temperature	Level
Separate(settle, filter, centrifuge)	Composition
React	Corrode

Remembering that the technique is called Hazard and operatibility studies, added to the above might be relevant operational words such as:

Isolate	Drain
Vent	Purge
Inspect	Maintain
Start up	Shut down

SECONDARY KEY WORDS:-

 As mentioned above, when applied in a conjunction with a primary key word, these suggest the potential deviations or problems. They tend to be a standard set as listed below.

Word	Meaning
No	The design intent does not occur(e.g.flow), or the operational aspect is not achieved (isolate/No)
Less	A quantitative decrease in the design intent occur(e.g. Pressure/Less)
More	A quantitative increase in the design intent occur(e.g. Temperature/More)
Reverse	The opposite of the design intent occurs(e.g. Flow/Reverse)
Other	The activity occurs, but not in the way intended(e.g. Flow/other) could indicate a leak or product flowing where it should not, or composition/other might suggest unexpected proportions in a feedstock
As well as	A quantitative increase
Part of	A quantitative decrease

HAZOP STUDY METHODOLOGY:-

In simple terms the HAZOP study process involves applying in a systematic way all relevant key words combination to the plant in question in an effort to uncover potential problems. The results are recorded in a columnar format under the headings,

Deviation

Cause

Consequences

Safeguard

Action

In considering the information to be recorded in each of these columns is given below.

DEVIATION

 The key words combination being applied (e.g. Flow/No)

CAUSE

Potential cause which would result in the deviation occurring (e.g. “strainer blockage due to impurities in Dosing tank” might be a cause of Flow/No).

CONSEQUENCES

 The consequences which would arise, both from the effect of the deviation (e.g.”Loss of dosing results in complete separation”) and, if appropriate from the cause itself (e.g. “cavitations in pumps, with possible damage if prolonged”).

SAFEGUARDS

 Any existing protective devices, which either prevent the cause or safeguards against the adverse consequences, would be recorded in this column. For example you may consider recording “Local pressure gauge in discharge from pump might indicate problem was arising”. Note that safeguard need not be restricted to hardware……where appropriate; credit can be taken for procedural aspects such as regular plants inspections(if you are sure that they will actually be carried out!).

ACTION 

Where a credible cause results in a negative consequence, it must be decided whether some action should be taken. It is at this stage that consequences and associated safeguards are considered. If it is deemed that the protective measures are adequate, then no action need to taken, and words to that effects are recorded in the action column.

Actions fall into two groups:

• Action that remove the cause
• Actions that mitigate or eliminate the consequences

Where as former is to be preferred, it is not always possible especially when dealing with equipments malfunction. However, always investigate removing the cause first and only where necessary mitigate the consequences.

Finally always take into account the label of training and experience especially of personnel who will operate the plant. Actions, which call for elaborate and sophisticated protective systems, are wasted, as well as being inherently dangerous, if operators do not and never will, understand how they function. It is not unknown for such devices to be disabled, either deliberately or in error, because no one knows how to maintain and calibrate them.

Having gone through the operations involve in recording a single deviation, these can now be put into the context of the actual study meeting procedure. From the flow diagram below it can be seen that it is very much an iterative process, applying in a structured and systematic way the relevant keyword combinations in order to identify potential problems.

HAZOP STUDY OF A SHELL AND TUBE EXCHANGER

In this study, pressure, temperature and level will be used as variables (secondary words) different guide words (primary words) will be applied to these words.

Pressure

Deviation 1:

Shell side very high pressure

Cause:

Exchanger outlet discharge SDV closes.

Consequence:

Exchanger shell side will be over pressurized.

Action Recommended:

High pressure security must be installed on shell outlet which if actuated will close all SDV’s.

Deviation 2

Low pressure on shell side.

Cause:

Feed pump trips

Consequence:

No significance effect as system remains within the design limits

Action Recommended:

Not available

Deviation 3

Tube side high pressure.

Cause:

Tube ruptures.

Consequences:

Tube may over pressurize, but since shell and tube side are designed at same pressure hence no significant effect is there.

Action Recommended:

High pressure security must be installed on tube outlet which if actuated will close all SDV’s.

Temperature

Deviation 4

Tube side high temperature.

Cause:

SDV on tube side outlet is suddenly closed

Action recommended:

 High temperature security must be there that will close tube side SDV’s

Flow

Deviation 5

No flow on shell side

Cause:

Shell side outlet SDV closes suddenly

Consequences:

Flow through compressor will reduce and tube side temperature will shoot up.

Action Recommended:

Low flow security to be installed on pump discharge which if actuated will trip feed pump and can trip the whole plant.